Research progresses by asking the right questions. While research into photonic systems such as metamaterials has rapidly gained momentum over the past two decades, one of the key questions now – as posed by many on the discussion webinar panel – is what to do with them.

“From my experience, research in metamaterials is very intensive, both computationally and to an even greater extent experimentally,” said Sergei Zhukovsky, an industrial researcher at Prism Valuation Inc. (Toronto, Canada), and co-affiliated with ITMO University (St. Petersburg, Russia). “So it is really about finding an application where it makes sense to invest this much.”

Tuning in on plasmonics and metamaterials

Metamaterials and plasmonics featured strongly in work presented by the panel, such as the study by Michele Tamagnone at École Polytechnique Fédérale de Lausanne and colleagues using an array of graphene nanoribbons to bend light. “The application here is very general,” he suggested. “Any application that requires modification of the wavefront – such as beam shaping and holograms – all these can be envisioned with this structure.”

The results hinge on the tunability of graphene, a property also exploited by Mohamed Farhat of King Abdullah University of Science and Technology in Saudi Arabia and his colleagues in work on a perfect broadband graphene metascreen absorber. “Using only metals, it is not possible to have this broadband response because the electrical properties cannot be tuned,” explained Farhat. “So this kind of absorber based on graphene and a high permittivity dielectric is the only way to have this broadband absorption using a very thin structure.”

Despite the challenges in studying these systems, Zhukovsky also stressed the important role metamaterials have played as a systematic approach to investigating inhomogeneous structured materials. These systems are far less well understood than the homogenous materials, and random media that have already been explored for several decades or more.

His recent research has examined multilayer systems of alternating high and low-index materials, revealing a breakdown in the effective medium approximation, both theoretically and experimentally, for systems just 20–40 layers deep. The layers are thin enough to allow light to tunnel through a high-index layer by means of the low-index layers, so that there is some transmission for the real structure, whereas there is none according to the effective medium approximation.

Nanofabrication – a delicate issue

“Because we are using scanning tunnelling luminescence, our experiment needs very clean surfaces, so surface fabrication is very important,” said Hiroshi Imada from RIKEN. He discussed some of his results probing GaAs surfaces with scanning tunnelling luminescence to gain a better understanding of the energy dissipation mechanisms at play – knowledge that may lead to improved luminescence efficiencies.

Progress in fabrication techniques has arguably played a significant role in allowing research in many strands of nanophotonics to progress, but realising these systems remains far from trivial. “If graphene metamaterials can be probed with our method, it could be very interesting, but the surface quality is the issue,” he added.

Nanofabrication obstacles were also noted in other areas of graphene research. “The main challenge for two-dimensional materials in general is that they are more fragile – you have to be careful because if you scratch it you lose it,” said Tamagnone. “One of the most important challenges is how to improve lithography on graphene.”

The Nanotechnology discussion webinar on nanophotonics was organized as part of the celebrations of the 2015 International Year of Light. You can hear the discussion in full on Nanotechnology, and read the research papers discussed in the Focus on Nanophotonics collection.